1. Field of the Invention
The present invention relates generally to the field of wireless communication systems and particularly to a method and apparatus for adaptive transmit beamforming with reduced overhead in multi input multi output (MIMO) communication systems.
2. Description of the Prior Art
Communication systems can be categorized as conforming to either wired or wireless standards. Implementations can range from local wireless networks in the home, to the national and international cell phone networks, to the worldwide Internet.
Each communication system deployed typically conforms to one or more of a number of existing standards. Wireless standards include the IEEE 802.11 wireless local area network (WLAN), the advanced mobile phone services (AMPS), Bluetooth, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution system (LMDS), multi-channel-multi-point distribution systems (MMDS), and various proprietary implementations of such standards.
Wireless devices in a network, such as a laptop computer, personal digital assistant, video projector, or WLAN phone, can communicate either directly or indirectly to other users or devices on the network. In direct communication systems, often referred to as point-to-point communication systems, the two devices are assigned one or more communication radio frequency (RF) channels, and the devices communicate directly over those channels. In indirect communication systems, the devices communicate through an intermediary device, such as an associated base station for cellular services, or an access point for home or office WLAN networking, on an assigned channel. To complete the connection, the access point or base station communicates with the pair directly, using the system controller, the Public switch telephone network (PSTN), the Internet, or some other wide area network.
Referring now to FIG. 1, a conventional WLAN home network is shown to include an access point (AP) or base station 200, a network interface hardware 204 and various electronic devices 206-214 that are used in a typical WLAN home network. The devices include a WiFi phone 206, a personal digital assistant (PDA)/WiFi camera 208, a laptop 210, a home audio system 212 and a high definition television (HDTV)/projector 214. The devices 206-214 communicate with each other through the AP 200 on assigned channels. The AP 200 has a beamforming capable transmission module 216 and a multi input multi output (MIMO) antenna 218. Each of the devices 206-214 also has a beamforming capable transmission module and an MIMO antenna. The AP 200 is connected to the Internet Wide area network (WAN) or LAN through the network interface hardware 204.
The commonly known architecture for a digital wireless device to receive data in one of these networks includes an antenna connected to an RF signal processing circuit. The antenna receives the RF signal and supplies it to the RF circuit, which filters out unwanted signal and noise from adjacent channels and in turn converts it to the baseband (i.e., centered at zero frequency, DC), or some intermediate frequency (IF). The analog RF output signal, at IF or baseband is converted to a digital stream and processed by the baseband module. The baseband module demodulates and decodes the baseband signal, thus recovering the original data.
Similarly, to send data over the network, the digital baseband encodes the bit stream, modulates the encoded stream, and if necessary converts it to an IF signal. The digital signal is converted to an analog signal using a digital to analog converter and sent to the RF circuit. The RF circuit converts the analog baseband output to an RF signal, using a carrier frequency corresponding to the channel assigned to the particular user, and sends the signal over the wireless channel using the RF antenna.
Traditional devices, particularly early (pre-2005) WLAN products adhering to the 802.11 standard, have used a single physical RF antenna, for both transmitting and receiving data. The antenna is shared by virtue of time-domain duplexing (TDD), whereby the transceiver only transmits or receives data at any given time, and does not do both simultaneously. Thus the antenna can be shared between the receiver and transmitter functions. When only one antenna is used at each end of the communication link, the channel established is referred to as a single-input/single-output channel, or SISO. More advanced systems based on this standard employ multiple antennae, for both receiving and transmitting data. A basic two-antenna device that switches between antennae automatically based on received signal quality is using antenna-switching diversity. If the two-antenna device transmits on both antennae simultaneously, the channel is referred to as multiple-input/single-output (MISO), if the receiver side of the link only has one receiver antenna. Conversely, if the transmitter uses a single antenna, and the receiver uses two antennae, the channel is called a SIMO (single-input/multiple-output) channel.
Currently entering the market are MIMO products (multiple-input/multiple-output) devices, aimed at very high performance in throughput, range and link reliability. These products are in advance of the published standard, as the Institute of Electrical and Electronic Engineers (IEEE) has only recently completed Draft 2.0 of the 802.11n standard that formalizes the operation of multiple antennae WLAN systems. In particular, the standard defines the protocols and techniques that enable multiple antenna systems to interoperate so that maximum beamforming benefits can be achieved. For example, systems with multiple antennae can transmit and receive more than one data stream simultaneously using a combination of either time or spatial encoding functions. This can effectively double or triple the throughput, depending on the number of parallel streams.
To improve signal reception quality, the Draft 2.0 standard also includes several beamforming modes of operation. Basically, beamforming is a technique where an array of antennae are “directed” at a desired target or source by adjusting the relative gain and phase of the array elements. By adjusting the relative gain and phase of the elements, the antenna pattern, or beam, can be made to point in a favored direction for receiving or transmitting data, or to attenuate other directions in order to reduce an interference source. Prior art publications describing these methods are: A Primer on Digital Beamforming by Toby Haynes, Spectrum Signal Processing 1998, Digital beamforming basics, by Hans Steyskal, Journal of Electronic Defense 1986. The foregoing references describe the basic mathematics associated with forming beam patterns in order to focus an antenna array to better receive or transmit RF signals.
In the 802.11n Draft 2.0 standard, there are several methods of beamforming detailed that pertain to the orthogonal frequency domain multiplexing (OFDM) type of modulation. The OFDM modulation is multi-carrier, utilizing the inverse fast Fourier transform (IFFT) process to convert N individual data symbols into a time-domain signal for transmission. At the receiver, the time domain signal is blocked up into symbols and demodulated back into a vector N individual frequency domain symbols. The received signal y on each subcarrier can be expressed as y=Hx+n, and x=Qs, where s is the sent symbol, Q is a pre-multiplier, H is the channel between the transmitter and receiver, and n is the effective noise. For an MIMO link, the channel H is a matrix. For example, in a two transmitter and three receiver (2T3R) system, the H matrix is 3×2, and the received signal equation can be written as:
            [                                                  y              1                                                                          y              2                                                                          y              3                                          ]        =                            [                                                                      h                  11                                                                              h                  12                                                                                                      h                  21                                                                              h                  22                                                                                                      h                  31                                                                              h                  32                                                              ]                ⁡                  [                                                                      x                  1                                                                                                      x                  2                                                              ]                    +              [                                                            n                1                                                                                        n                2                                                                                        n                3                                                    ]              ,and the sent signal is expressed as:
      [                                        x            1                                                            x            2                                ]    =                    [                                                            q                11                                                                    q                12                                                                                        q                21                                                                    q                22                                                    ]            ⁡              [                                                            s                1                                                                                        s                2                                                    ]              .  
During the course of decoding the received data, the channel matrix H is estimated, using the preamble portion of the data packet. To implement one particular form of beamforming, the channel state information (CSI) based on H is processed by the receiver and sent back to the transmitter. The transmitter then formulates a beamforming matrix Q, based on the CSI, and uses Q to beamform subsequent packets to the corresponding receiver. This process, by which the CSI is sent back to the transmitter for beamforming, is referred to as explicit beamforming.
Explicit beamforming is one method described in the 802.11n standard. There are three associated feedback formats for the CSI information. The first implementation utilizes a scaled version of the H matrix, and is referred to as full CSI. The data required to send the full CSI as feedback can be prohibitive because much dynamic range is required to represent a channel that has any significant fading. To alleviate this, another method referred to a steering matrix feedback is implemented. In this case the channel matrix is first decomposed using the singular value decomposition (SVD), so thatH=UΣV*  Eq. (1)After the decomposition, the V matrix is sent back to the transmitter. The advantage is that the V matrix is unitary (all its columns have unity norm), so that less resolution is needed to represent the channel, and thus there is reduced overhead with the method. The problem with this method is that small variations in the channel condition could lead to large changes to the V matrix. Thus, this method may not be robust and suffers from sudden performance loss in both throughput and link stability.
To further reduce the amount of information required for beamforming feedback, a third protocol is spelled out in the standard that compresses the channel state information. This is accomplished using Givens rotation decomposition and polar coordinates to parameterize the steering matrix, and feeding this back to the transmitter instead. Prior art limitations include feeding back an incomplete CSI to the transmitter; thereby limiting the type of beamforming processing that can be used, and, because it relies on the V matrix, it too may be sensitive to channel variations, as mentioned with the compressed method discussed above.
Thus, there exists a need to provide a method and apparatus for explicit feedback transmit beamforming that allows full and accurate CSI that requires a low amount of overhead to implement.